Distinguishing pathogenic from non-pathogenic microbes presents an interesting challenge to the host immune system and is critical for eliciting the appropriate immune response to combat the invading pathogen. Vacuolar pathogens have evolved multiple strategies to manipulate host functions to promote intracellular survival, necessitating co-evolution of immune molecular sensory networks that detect the pathogen attempts at subversion. To dissect how this process occurs at the molecular level we have developed methodology that monitors modifications on host proteins to reverse-engineer the host response networks to pathogenic infection in silico. To this end, we are using the respiratory pathogen Legionella pneumophila as a model organism to decipher specific regulation of host cellular pathways to pathogenic challenge. The overall goal of the project is to use biochemical and molecular approaches to characterize how cellular pathways in macrophages utilize regulatory molecular modifications, such as ubiquitin, to specifically alter the outcome of signaling cascades in response pathogenic infection. Specific emphasis is placed on the cellular cascades that coordinate the cytokine response of innate immune cells.